System and Method for Facilitating Homogeneous Charge Compression Ignition

ABSTRACT

A method of transitioning between homogeneous charge compression ignition and spark ignition in a combustion chamber of an engine that is configured for homogeneous charge compression ignition under at least some engine operating conditions and for spark ignition under at least some engine operating conditions, the method comprising heating contents of the combustion chamber with a glow plug prior to transitioning between homogeneous charge compression ignition and spark ignition.

BACKGROUND AND SUMMARY

Internal combustion engines convert chemical energy in a fuel tomechanical energy. As part of the conversion, the fuel can be combusted,thus causing hot combustion products to expand within the engine. Theexpansion of the combustion products can be used to move mechanicalcomponents of the engine, such as pistons. Combustion reactions can haveseveral products, or emissions, some of which can be undesirable. Forexample, when hydrocarbons are used as fuel, combustion products caninclude HC, CO, CO2 and NOx.

An internal combustion engine may operate in one or more combustionmodes. One example mode is spark ignition (SI), where an electric sparkfrom a sparking device is used to initiate combustion of an air and fuelmixture. Another example mode is homogeneous charge compression ignition(HCCI), where an air and fuel mixture achieves a temperature whereautoignition occurs without requiring a spark from a sparking device. Insome conditions, HCCI may have greater fuel efficiency, reduced NOxproduction, and/or other advantages compared to SI. However, in someconditions, such as with high or low engine loads and/or high or lowengine speeds, it may be difficult to achieve reliable HCCI combustion.

Numerous attempts have been made to design a dual combustion mode enginethat is configured to utilize SI during some conditions and HCCI duringother conditions. For example, U.S. Pat. No. 6,619,254 describes a dualcombustion mode engine that uses SI and HCCI. Further, a thirdcombustion mode is described where under certain operating conditionsthe pressure, temperature, and composition of the charge are set in sucha way that the self-ignition capability is just short of being reached,and an external energy source in the form of an electric spark or anadditionally injected quantity of fuel is used to trigger ignition.

The inventors herein have recognized disadvantages with previousattempts at HCCI operating mode engines, including dual mode enginesthat use SI combustion at least some of the time. Since SI combustion isgenerally hotter than HCCI combustion, when switching from SI operationto HCCI operation, there is a period when the temperature of combustionis decreasing with each combustion event and a hybrid SI-HCCI combustionoccurs. Similarly, when switching back into SI operation, thetemperature of combustion is expected to increase back up to SI levelsduring the hybrid combustion phase. This hybrid combustion is suboptimalin terms of stability, efficiency, and emissions generation. If not wellcontrolled, such a hybrid combustion can cause misfire, and in anextreme case, combustion can cease altogether.

Furthermore, if a spark assist is used during transition periods betweenSI and HCCI operation, some of the benefits of HCCI may not be fullyrealized. In particular, NOx emissions and/or fuel economy may be lessfavorable than if HCCI was used without spark assist.

Thus, it may be advantageous to improve transition control of a pluralcombustion mode engine (e.g., SI/HCCI), with or without using a sparkassist. In one approach, transition control may be addressed by heatingcontents of a combustion chamber with a device having a small thermalinertia (e.g., a glow plug) when transitioning between SI andhomogeneous charge compression ignition. In this way, it may be possibleto decrease SI or hybrid SI/HCCI in favor of HCCI, thus furtherrealizing benefits of HCCI.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary engine system including a heating device witha low thermal inertia for heating contents of a cylinder.

FIG. 2 schematically plots combustion mode suitability for differentengine operating conditions.

FIG. 3 schematically plots spark plug and glow plug control as engineoperating conditions change.

FIG. 4 is a flow chart showing an exemplary HCCI mode switching controlprocedure.

WRITTEN DESCRIPTION

The present disclosure is directed to a system and method forfacilitating homogeneous charge compression ignition (HCCI) in aninternal combustion engine. HCCI is different than the propagation modeof combustion. HCCI engines can introduce a homogeneous mixture of fueland air into a combustion chamber, where compression heating of thecharge leads to simultaneous (or near simultaneous) ignition throughoutsubstantially the entire charge. HCCI is thus different than a flamepropagation combustion mode, where ignition first occurs at a distinctpoint, and then forms a flame front that advances from the ignitionpoint.

According to some aspects of the disclosure, a heating device with asmall thermal inertia herein referred to as a glow plug, can be used toadd heat directly to a combustion chamber, thus facilitating HCCI.During some conditions, such as at low engine load and/or speed, thecooler engine operation may make it difficult to sustain HCCIcombustion, thus combustion may be phased later than desired and/ormisfire may occur. Heat added via a glow plug can be used to initiateautoignition by increasing the temperature of the mixture in thevicinity of the plug. Hence during conditions where the desiredautoignition conditions are difficult to achieve, combustion may bephased earlier and/or misfire may be avoided. In this manner, anextremely fast heat response may be achieved, thereby facilitatingoperation in HCCI. Further, this approach may be especially usefulduring other periods of low speed and/or low load operation, such asduring a gear change, to avoid switching into SI mode during the gearchange and then switching back into HCCI afterwards. In yet anotherapproach, this technique could also be used during steady state lowspeed and/or low load HCCI operation or other engine operatingconditions that may not otherwise be conducive to HCCI.

Unlike prior art engines, such as diesel engines, which have used glowplugs to cold start an engine, the present disclosure provides for theuse of glow plugs to heat a combustion chamber throughout differentphases of engine operation. Adding heat from an external source canprovide a stabilizing force during SI/HCCI mode switching and/or whenengine operating conditions are near boundary conditions where HCCIbegins to be suitable. Heat can be added during transition periodsbetween SI and HCCI. Heat can also be added during transitory periodswhere engine operating conditions temporarily are outside the bounds ofwhere HCCI can occur without assistance from a glow plug, but relativelyquickly return to an unassisted operating region (e.g., during gearchanges). Heat can also be added for sustained periods in which anengine operating condition is outside the bounds of where HCCI can occurwithout assistance from a glow plug, thus effectively increasing thesteady-state conditions in which HCCI is available. Because heat from aglow plug can be added relatively quickly, heat from a glow plug canserve as a short term facilitator of HCCI until another heat source,such as exhaust gas, has time to facilitate HCCI. Heat can be added toan engine that uses only homogeneous charge compression, or to an enginethat uses homogeneous charge compression and one or more other types ofignition, such as SI.

Transitions between HCCI and SI may be facilitated by varying one ormore parameters, including, but not limited to: air/fuel ratio, throttleposition, amount of fuel injected, timing of fuel injection(s), numberof fuel injections performed during a cycle, intake and/or exhaust valvetiming, spark timing, contribution of EGR, EGR cooling, intake airheating/cooling, turbocharging/supercharging, adding heat via a glowplug or other heating source, etc.

FIG. 1 schematically shows a direct injection gasoline engine 24 thatincludes a glow plug 26 that is configured to selectively add heatdirectly to a combustion chamber 29. Combustion chamber 29 is partiallydefined by cylinder walls 31 and piston 35. The piston is connected tocrankshaft 39. Combustion of air and fuel mixtures within the combustionchamber can cause the pistons to move, and movement of the pistons canbe translated to other components, such as crankshaft 39.

As will be described in more detail below, combustion in engine 24 canbe of various types, depending on a variety of conditions. SI may beused where the engine utilizes a sparking device to perform a spark sothat a mixture of air and fuel combusts. HCCI may be used where asubstantially homogeneous air and fuel mixture attains an autoignitiontemperature within the combustion chamber and combusts without requiringa spark from a sparking device. Other types of combustion may bepossible. For example, the engine may operate in a spark assist mode,wherein a spark is used to indirectly initiate autoignition of a part ofan air and fuel mixture and residuals. The engine may also operate inother compression ignition modes, wherein a mixture of air, fuel andresiduals are not necessarily homogeneous. Furthermore, and as isdescribed in more detail below, heat can be added to a cylinder (withoutspark) to facilitate reaching an autoignition temperature wherehomogeneous charge compression occurs. The above listed combustion modesare non-limiting examples, and additional or alternative modes can beused.

As used herein, a “glow plug” can be virtually any device and/orassembly adapted to add heat directly to a cylinder. A glow plug can bea resistive element that increases temperature responsive to an appliedvoltage/current. A glow plug can be configured to increase temperaturewithout creating a spark or other type of open flame that wouldprematurely ignite a charge, thus undesirably preempting HCCI. In otherwords, a glow plugs heating mechanism can differ from that of the sparkplug, and therefore facilitate a different type of combustion than thespark plug. Depending on a chosen configuration, a glow plug can heat acylinder from one or more specific locations, or the glow plug can heata cylinder more evenly throughout the cylinder. For example, a glow plugcan be a plug-shaped device positioned at a particular location in thecylinder, or the glow plug can be applied to and/or integrated with oneor more portions of the cylinder wall to provide more distributedheating. In some embodiments, two or more glow plugs can be used in acylinder.

In FIG. 1, glow plug 26 is illustrated as a plug shaped device locatednear the top of the combustion chamber, although such a configurationand/or positioning is not required. In some embodiments, glow plug 26may include a combustion sensor for detecting the timing of thecombustion and/or whether combustion has occurred. By detectingcombustion within the combustion chamber, HCCI operation may be assessedand the autoignition timing may be adjusted as desired, for example, viaheat added by the glow plug and/or an adjustment of one or more otheroperating conditions. In some embodiments, combustion chamber 29 mayinclude a dedicated combustion sensor 100 for detecting the occurrenceand/or timing of combustion. Combustion sensor 100 or other combustionsensing devices may detect combustion by sensing the peak pressurewithin the combustion chamber. As will be described herein in greaterdetail, the detection of combustion timing may be used as feedback forglow plug control.

To initiate SI, engine 24 includes a distributorless ignition system 88that provides ignition spark to combustion chamber 29 via spark plug 92,although other sparking arrangements are within the scope of thisdisclosure. A control system 48 can be used to command spark plug 92, oranother sparking device, to spark at desired times. In some embodiments,spark plug 92 may be operated as a combustion sensor. For example, sparkplug 92 may be used to detect various products of combustion such asions within the combustion chamber using an approach known as ionsensing. The control system can cooperate with a plurality of sensors,examples of which are described below, to assess engine operatingconditions suitable for SI. Assessing engine operating conditionssuitable for SI may at least partially include assessing engineoperating conditions that are not suitable for HCCI. In other words,control system 48 may attempt to maximize the time that engine 24 usesHCCI, resorting to SI when engine operating conditions are not suitablefor HCCI. Nonlimiting examples of parameters that may be monitored toassess whether engine operating conditions are suitable for SI includeengine speed, engine load, combustion chamber temperature, manifoldpressure, and intake air temperature. In some embodiments, combustionchamber temperature may be assessed by and/or deduced from at least oneof a detection of the cylinder wall temperature via a sensor such as112, temperature of the gasses and/or ions within the combustion chambervia a sensor disposed therein (e.g. ion sensor, temperature sensor,etc.), temperature of the exhaust gases flowing from the combustionchamber via a temperature sensor within the exhaust passage and/ordetection of various combustion products, etc.

During SI mode, the temperature of intake air entering the combustionchamber may be near ambient air temperature and therefore may besubstantially lower than the temperature required for autoignition ofthe air and fuel mixture. Since a spark is used to initiate combustionin SI mode, control of intake air temperature may be more flexible ascompared to HCCI mode. Thus, SI mode may be utilized across a broadrange of operating conditions (such as higher or lower engine loads),however SI mode may produce different levels of emissions and fuelefficiency under some conditions compared to HCCI combustion.

In some conditions, during SI mode operation, engine knock may occur ifthe temperature within the combustion chamber is too high. Thus, underthese conditions, engine operating conditions may be adjusted so thatengine knock is reduced, such as by retarding ignition timing, reducingintake charge temperature, varying combustion air-fuel ratio, orcombinations thereof.

During HCCI mode operation, the air/fuel mixture may be highly dilutedby air and/or residuals (e.g. lean of stoichiometry), which results inlower combustion gas temperature. Thus, at least some engine emissionsmay be substantially lower than SI combustion under some conditions.Further, fuel efficiency with autoignition of lean (or diluted) air/fuelmixture may be increased by reducing the engine pumping loss, increasinggas specific heat ratio, and/or by utilizing a higher compression ratio.During HCCI combustion, autoignition of the combustion chamber gas maybe controlled so as to occur at a prescribed time so that a desiredengine torque is produced. Since the temperature of the intake airentering the combustion chamber may be relevant to achieving the desiredautoignition timing, operating in HCCI mode at high and/or low engineloads may be difficult.

FIG. 2 schematically shows a graphic representation of a nonlimitingexample of engine operating conditions that are suitable for HCCI. Inparticular, engine speeds and engine loads that are suitable for HCCIare indicated at 202. While HCCI operating region 202 is schematicallyshown as a rectangle, it should be understood that a more complexrelationship between engine load and engine operating speed may provideconditions suitable for HCCI. Furthermore, variables other than engineload and engine speed may affect which engine operating conditions aresuitable for HCCI.

As is schematically shown in FIG. 2, HCCI operating region 202 does notextend to low engine speeds or low engine loads. A glow plug can be usedto add heat to a combustion chamber, thus extending the range of engineoperating conditions suitable for HCCI. A glow plug assisted HCCI regionis schematically shown at 204. While glow plug assisted HCCI regionextends the low engine speed and/or low engine load engine operatingconditions suitable for HCCI, it should be understood that a glow plugmay additionally or alternatively be used to extend the high enginespeed and/or high engine load engine operating conditions suitable forHCCI, as indicated in dashed lines at 206. Spark ignition can be usedfor engine operating conditions that are outside the bounds of HCCIoperating region 202 and glow plug assisted HCCI region 204 (and/or206), as indicated by SI operating region 208. Furthermore, sparkignition can be used for engine operating conditions that are suitablefor HCCI and/or glow plug assisted HCCI, if desired.

FIG. 3 shows combustion mode plotted as engine operating conditionschange. Area above the HCCI THRESHOLD line represents engine operatingconditions that are suitable for HCCI. Area below the HCCI THRESHOLDline represents engine operating conditions that are not suitable forHCCI. As used with reference to FIG. 3, “engine operating conditions”can represent one or more parameters, such as engine speed, engine load,gear changes, combustion chamber temperature, and/or others. Line 300schematically represents the various factors that are collectivelydescribed as the engine operating conditions. When the line is above theHCCI THRESHOLD, conditions are suitable for HCCI. When the line is belowthe HCCI THRESHOLD, conditions are suitable for spark ignition.

A glow plug can be used to improve transition between SI and HCCI and/orvice versa. As an example, when conditions for HCCI operation are met(e.g., the HCCI speed/load window of operation has been entered), theglow plug can be activated. When enough time for the glow plug to takeaffect has elapsed, a mode switch from SI to HCCI can be performed.Then, when stable HCCI is achieved, the glow plug can be deactivated.

A similar process flow can be performed when switching from HCCIoperation to SI operation. In particular, when engine operatingconditions have reached, or are nearing, an SI operating region, a glowplug can be activated during the transitory period in which spark plugoperation begins.

Furthermore, a similar control flow logic can be used for mode switchesthat occur during HCCI operation, for example, when switching betweendifferent HCCI fuel injection strategies. As a nonlimiting example, aglow plug can be used during transitions between a first HCCI injectionstrategy and a second HCCI injection strategy.

FIG. 4 is a flow chart that schematically illustrates a nonlimitingcontrol strategy for heat assisted HCCI mode switching. At 402, thepresent example begins with SI operation and no heat assist. At 404,engine operating conditions are assessed to determine whether conditionsare suitable for HCCI operation. As with other decisions used in thecontrol strategy, such an assessment can be performed by a controlsystem cooperating with one or more sensors. If conditions are notsuitable for HCCI, SI is continued. If conditions are suitable for HCCI,a heat source is activated to improve the transition between SI andHCCI, as shown at 406. At 408, it is determined whether a mode switch isfeasible. If a mode switch is not feasible, SI operation continues witha heat source assist. If a mode switch is feasible, HCCI operationbegins at 410. At 412 it is determined whether stable HCCI operation hasbeen achieved. Stable HCCI operation may be assessed, for example, whenthe desired autoignition timing is detected via a combustion sensorand/or via an ion sensor. If stable HCCI operation has not beenachieved, HCCI operation may continue with a heat source assist. Ifstable operation has been achieved, the heat source can be deactivatedat 414. At 416 it is determined if conditions for SI are present. Ifnot, HCCI operation continues. If conditions are suitable for SI, theheat source is activated at 418. At 420, it is determined whether a modeswitch is feasible. If not, HCCI operation may continue with a heatsource assist. If a mode switch is feasible, SI operation begins at 422.At 424 it is determined if stable SI operation has been achieved. Ifnot, SI operation continues with a heat source assist. If stable SIoperation exists, the heat source is deactivated at 402.

The following is a nonlimiting example of an engine system in which theabove described HCCI controls can be implemented. While the descriptionuses engine 24 as an example, it should be understood that the HCCIcontrols and/or the heat assists described herein can be implemented ina variety of differently configured engine systems. Turning now to FIG.1, combustion chamber 29 is shown communicating with intake manifold 43and exhaust manifold 47 via respective intake valve 52 and exhaust valve54. While only one intake and one exhaust valve are shown, the enginemay be configured with a plurality of intake and/or exhaust valves.Furthermore, while one cylinder is illustrated in FIG. 1, an engine caninclude virtually any number of cylinders (e.g., 3 cylinders, 4cylinders, 6 cylinders, 8 cylinders, 10 cylinders, 12 cylinders, etc.).

Universal Exhaust Gas Oxygen (UEGO) sensor 76 is shown coupled toexhaust manifold 47 upstream of a catalytic converter 70. Exhaust gassensor 76 is shown coupled to exhaust manifold 47 upstream of catalyticconverter 70. The signal from sensor 76 can be used to advantage duringfeedback air/fuel control to maintain average air/fuel at (or very near)stoichiometry during the stoichiometric homogeneous mode of operation.

Engine 24 can include an exhaust gas recirculation (EGR) systemconfigured to supply exhaust gas to intake manifold 43 from exhaustmanifold 47 via EGR passage 131. The amount of exhaust gas supplied bythe EGR system can be controlled by EGR valve 134. Further, the exhaustgas within EGR passage 131 may be monitored by an EGR sensor 133, whichcan be configured to measure temperature, pressure, gas concentration,etc. Under some conditions, the EGR system may be used to regulate thetemperature of the air and fuel mixture within the combustion chamber,thus providing a method of controlling the timing of autoignition forHCCI combustion. An EGR system is not required in all embodiments.

In some embodiments, engine 24 can include a boosting device such as aturbocharger or a supercharger. For example, a turbocharger having acompressor communicatively coupled within the intake passage upstream ofthe engine and may be used during some operations to increase thepressure of the intake air provided to one or more cylinders of theengine. However, in some conditions, boosting may complicate temperaturecontrol of the combustion chamber, thereby making autoignition timingcontrol more difficult. As such, in some embodiments, the use of a glowplug to provide selective charge heating may be varied in response tothe amount of boost provided by a turbocharger and/or a supercharger, atleast under some conditions.

In some embodiments, valve control may be provided by variable camtiming (VCT), cam profile switching (CPS), and/or variable valve lift(VVL); however other methods may be used such as electrically controlledvalves. FIG. 1 shows intake valve 52 and exhaust valve 54 controlled bycam shafts 130 and 132, respectively. Further, the valve timing (e.g.timing of opening and closing of valves) and/or valve lift (e.g. liftheight and lift duration) may be varied via actuators 136 and 138, basedon operating conditions. The actuators may be hydraulically powered, orelectrically actuated, or combinations thereof to provide for variablecam timing and/or cam profile switching in response to signals receivedfrom the control system. For example, signal line 150 can send a valvecontrol signal including a cam timing signal and/or cam selection signalto unit 136 and receive a cam timing measurement. Likewise, signal line152 can send a valve control signal including a cam timing signal and/orcam selection signal to unit 138 and receive a cam timing measurement.While in this example, independent intake cam timing and exhaust camtiming are shown, variable intake cam timing may be used with fixedexhaust cam timing, or vice versa. Also, various types of variable valvetiming may be used, such as the hydraulic vane-type actuators from acontrol system 48.

In some embodiments, cam actuated exhaust valves may be used withelectrically actuated intake valves, if desired. In such a case, thecontrol system can determine whether the engine is being stopped orpre-positioned to a condition with the exhaust valve at least partiallyopen, and if so, hold the intake valve(s) closed during at least aportion of the engine stopped duration to reduce communication betweenthe intake and exhaust manifolds. In addition, intake manifold 43 isshown communicating with an electronic throttle 125. As with otherexemplary components illustrated in FIG. 1, an electronic throttle 125is not required in all embodiments.

Engine 24 includes a fuel injector 65 for delivering liquid fueldirectly to combustion chamber 29 in proportion to the pulse width ofsignal Fpw from control system 48. As shown, the engine may beconfigured such that the fuel is injected directly into the enginecylinder, which is known to those skilled in the art as directinjection.

Control system 48 is shown in FIG. 1 as including a conventionalmicrocomputer including: microprocessor unit 102, input/output ports104, and read-only memory 106, random access memory 108, keep alivememory 110, and a data bus. A control system can include more than oneof any of the above described components, and/or any alternative oradditional components that can be used to execute monitoring,decision-making, and/or control instructions. Control system 48 is shownreceiving various signals, including signals from sensors coupled toengine 24. Exemplary signals include engine coolant temperature (ECT)from temperature sensor 112 coupled to cooling sleeve 114; a pedalposition sensor 119 coupled to an accelerator pedal; a measurement ofengine manifold pressure (MAP) from pressure sensor 122 coupled tointake manifold 43; a measurement of engine air charge temperature ormanifold temperature from temperature sensor 117; and an engine positionsensor from a Hall effect sensor 118 sensing crankshaft 39 position. Insome embodiments, the requested wheel output can be determined by pedalposition, vehicle speed, and/or engine operating conditions, etc. In oneaspect of the present description, engine position sensor 118 produces apredetermined number of equally spaced pulses every revolution of thecrankshaft from which engine speed (RPM) can be determined.

Engine 24 includes an aftertreatment system comprising a catalyticconverter 70 and a lean NOx trap 72. In this particular example,temperature Tcat1 of catalytic converter 70 is measured by temperaturesensor 77 and temperature Tcat2 of lean NOx trap 72 is measured bytemperature sensor 75. Further, gas sensor 73 is shown arranged inexhaust passage 47 downstream of lean NOx trap 72, wherein gas sensor 73can be configured to measure the concentration of NOx and/or O2 in theexhaust gas. Lean NOx trap 72 may include a three-way catalyst that isconfigured to adsorb NOx when engine 24 is operating lean ofstoichiometry. The adsorbed NOx can be subsequently reacted with HC andCO and catalyzed when control system 48 causes engine 24 to operate ineither a rich homogeneous mode or a near stoichiometric homogeneous modesuch operation occurs during a NOx purge cycle when it is desired topurge stored NOx from the lean NOx trap, or during a vapor purge cycleto recover fuel vapors from fuel tank 160 and fuel vapor storagecanister 164 via purge control valve 168, or during operating modesrequiring more engine power, or during operation modes regulatingtemperature of the omission control devices such as catalyst 70 or leanNOx trap 72. Various different types and configurations of emissioncontrol devices and purging systems may be employed.

In some embodiments, engine 24 while operating in HCCI mode maytransition to SI mode to purge the lean NOx trap. Since it may bedesirable to reduce transitions between combustion modes, the conditionof the lean NOx trap can be considered in conjunction with engineoperating conditions before performing a transition. Alternatively, insome embodiments, the lean NOx trap may be purged, irregardless of orindependent from the condition of the lean NOx trap, prior totransitioning to HCCI mode to maximize the capacity of the trap therebyfurther reducing future engine transitions. The condition of the leanNOx trap may be inferred through estimation based on engine past orpredicted engine operating conditions and/or may be measured by sensor75 or NOx sensor 73, for example. If it is determined or predicted thatthe capacity of the lean NOx trap needs purging, the engine maytransition from HCCI mode to SI mode or remain in SI mode, wherein theengine is temporarily operated at stoichiometry or rich of stoichiometryto purge the lean NOx trap. In some embodiments, transitions betweencombustion modes may include an intermediate combustion mode, whichcould include any of the combustion modes described herein among others.

In some embodiments, the control system may be configured to transitionthe engine from HCCI mode to SI mode when the temperature of the exhaustaftertreatment system (catalyst 70 and/or lean NOx trap 72, amongothers) is determined to be too low (i.e. lower than a threshold), sincethe HCCI exhaust temperature can be substantially lower than the SIexhaust temperature under some conditions. Alternatively, in someembodiments, a transition from SI mode to HCCI mode may be performedwhen the exhaust temperature is determined to be too high and/or higherthan a threshold. Further, in some embodiments, a transition betweencombustion modes may be based at least partially on a condition of theheating ventilation and air conditioning (HVAC) system. For example,during some conditions, HCCI mode may not provide sufficient passengercabin heat in cold ambient conditions. In another example, if an airconditioning (A/C) compressor is operated, the additional engine outputrequired to operate the compressor may cause the engine output to becomegreater than an upper limit of the HCCI operating range. Further, insome embodiments, a transition between combustion modes may be based atleast partially on a condition or state of the transmission. Forexample, when the torque converter is in a locked configuration, thehigh torque pulsations and high rate of pressure rise from HCCIoperations may be less acceptable than when the torque converter is inan unlocked configuration. In other words, the operating range for HCCImode may be different depending on whether the converter is locked orunlocked. Similarly, different gears or gearing configurations withinthe transmission may be more sensitive to noise and vibration harshness(NVH) during HCCI mode. Thus, it should be understood that the controlsystem may be configured to transition the engine between combustionmodes based on any of the above mentioned conditions.

As described above with reference to FIG. 1, engine 24 may include afuel vapor purge system comprising fuel tank 160, fuel vapor storagecanister 164, and purge control valve 168 fluidly coupled to intakemanifold 43. In some embodiments, the internal combustion engine can beconfigured to operate in a first purge state, in which fuel vapors arepermitted to be received from the fuel vapor purge system only intocombustion cylinders that are operating in the spark ignition mode, andin a second purge state, in which fuel vapors are permitted to bereceived from the fuel vapor purge system into combustion cylindersoperating in the spark ignition mode and into combustion cylindersoperating in the HCCI mode. Such an engine provides the benefits ofmultiple combustion modes while making efficient use of evaporated fuelvapors. Further, it is possible to reduce uncertainties in auto-ignitiontiming, thereby enabling improved HCCI operation.

1-16. (canceled)
 17. A method of operating an engine, the methodcomprising: activating a spark plug for one or more spark ignitioncombustion events; then deactivating the spark plug for one or more HCCIcombustion events, then activating a heating device for one or morecombustion events when engine operating conditions are outside a boundsof unassisted HCCI combustion while said spark plug is deactivated; andthen deactivating the heating device for one or more unassisted HCCIcombustion events.
 18. The method of claim 17, wherein said heatingdevice is a glow plug, and wherein said glow plug is in a combustionchamber of said plural-ignition-mode engine.
 19. The method of claim 17,further comprising a second heating device said second heating deviceactivated for one or more combustion events when engine operatingconditions are outside said bounds of unassisted HCCI combustion whilesaid spark plug is deactivated.
 20. The method of claim 17, furthercomprising purging a NOx trap during said unassisted HCCI combustionevents.
 21. The method of claim 17, wherein said heating device isactivated during a gear change.
 22. The method of claim 17, furthercomprising recognizing stable HCCI combustion, wherein the heatingdevice is deactivated after stable HCCI combustion is recognized. 23.The method of claim 22, wherein recognizing stable HCCI combustionincludes assessing engine speed.
 24. The method of claim 22, whereinrecognizing stable HCCI combustion includes assessing engine load. 25.The method of claim 22, wherein recognizing stable HCCI combustionincludes assessing combustion chamber temperature.
 26. The method ofclaim 18, further comprising reactivating the glow plug for one or morehybrid HCCI/SI combustion events prior to returning to unassisted SI.27. A method of transitioning between homogeneous charge compressionignition and spark ignition in a combustion chamber of an engine, themethod comprising: during a gear change heating contents of thecombustion chamber with a glow plug prior to transitioning between sparkignition and homogeneous charge compression ignition; and adjusting atleast one of a air/fuel ratio, throttle position, amount of fuelinjected, timing of fuel injection, number of fuel injections performedduring a cycle, intake and exhaust valve timing, spark timing,contribution of EGR, EGR cooling, intake air temperature.
 28. The methodof claim 27, wherein heating contents of the combustion chamber with aglow plug prior to transitioning between homogeneous charge compressionignition and spark ignition includes heating the combustion chamberduring at least part of an interval between a last homogeneous chargecompression ignition and a first spark ignition.
 29. An engine,comprising: a combustion chamber; a spark plug providing ignition sparkto said combustion chamber; a heating device for adding heat directly tosaid combustion chamber; a control system including instructions forHCCI combustion during a first operating condition, said control systemincluding instructions for HCCI combustion and activating said heatingdevice while said spark plug is deactivated during a second operatingcondition.
 30. The engine of claim 29, wherein said control systemfurther includes instructions for combusting an air-fuel mixture whilesaid spark plug is activated during a third operating condition.
 31. Theengine of claim 29, wherein the control system includes at least onesensor to assess engine speed, and wherein assessed engine speed atleast partially indicates suitability for spark ignition and homogeneouscharge compression ignition.
 32. The engine of claim 29, wherein thecontrol system includes at least one sensor to assess engine load, andwherein assessed engine load at least partially indicates suitabilityfor spark ignition and homogeneous charge compression ignition.
 33. Theengine of claim 29, wherein the control system includes at least onesensor to assess combustion chamber temperature, and wherein assessedcombustion chamber temperature at least partially indicates suitabilityfor spark ignition and homogeneous charge compression ignition.
 34. Theengine of claim 29, further comprising an exhaust system including a NOxtrap.
 35. The engine of claim 29, further comprising an injector forinjecting fuel directly into said combustion chamber.
 36. The engine ofclaim 29, wherein said control system includes instructions forassessing engine conditions suitable for HCCI combustion.